Tycho Brahe was the last major observational astronomer to
work without a telescope; he used sextants and quadrants. Galileo
Galilei introduced telescopes into astronomy in 1610. Johannes
Kepler tried to identify causes for his laws of planetary motion
but did not have the benefit of the teachings of Galileo or Isaac
Newton. Galielo taught why the planets continue to orbit with no
force to push them, and Newton taught that gravity explained why
orbits are elliptical. Kepler’s idea that the earth
attracted the moon like a magnet came from William Gilbert.